Of the hydroxybenzaldehydes obtained by the process of the present invention, salicylaldehyde and p-hydroxybenzaldehyde are both an industrially important compound, the former being useful as an intermediate for perfumes, agricultural chemicals and chelating agents, and the latter as an intermediate for the preparation of agricultural chemicals and medicines (penicillin modifiers, etc.).
The following methods are, for example, so far known as the preparation of hydroxybenzaldehydes:
(1) So-called Conventional Reimer-Tiemann Reaction: PA0 (2) Method prepared by Yoel Sasson et al [Tetrahedron Lett., 3753 (1979) and 1875 (1980)]: PA0 (3) A method in which salicylaldehyde is prepared in good yield by one step reaction using as starting materials a phenol and formaldehyde and a specific catalyst (a base represented by a tertiary amine and/or an organometallic salt): PA0 (4) A method for selectively obtaining p-hydroxybenzaldehyde by oxidizing p-cresol derivatives with oxygen:
In accordance with this reaction, a phenol is reacted with chloroform and an aqueous alkali solution in a heterogeneous system to prepare a mixture of salicylaldehyde and p-hydroxybenzaldehyde. However, in this method, the yield of salicylaldehyde is generally low. Further, an excess amount of chloroform over the phenol is necessary, and the recovery and recycling of unreacted chloroform and phenol are difficult.
As one of typical examples of improved Reimer-Tiemann reaction method, a method wherein aqueous methanol containing 10 to 75% by weight of methanol is employed as a reaction medium is known (as disclosed in U.S. Pat. No. 3,365,500). Although this method has an advantage that the amount of formed tar (high-boiling substance) is reduced, it has so defects that the conversion of phenol is low, the separation and recovery of unreacted phenol are difficult, and the formation ratio of salicylaldehyde to p-hydroxybenzaldehyde is lower than that obtained under the ordinary Reimer-Tiemann condition. Therefore, this method accompanies difficulties in the separation of the components from the reaction mixture and the like.
In accordance with this method, when an aliphatic tertiary amine is added to the reaction mixture obtained under the ordinary Reimer-Tiemann condition (where excess amounts of chloroform and an aqueous alkali solution are used over the phenol), the yield of salicylaldehyde is improved without any undesirable influence over the yield of p-hydroxybenzaldehyde. But this method accompanies not a few disadvantages. Namely, the tertiary amine which can be used is limited to a few species such as (n-C.sub.4 H.sub.9).sub.3 N, etc., and a reaction to form an alkyl ether of phenol also takes place competitively. Further, excess amounts of chloroform and an aqueous alkali solution over the phenol need be used, which leads to the foregoing disadvantages. Still further, it has been experimentally confirmed by the present inventors under the same conditions as reported that the effects brought by the addition of the aliphatic tertiary amine are not so drastic as reported.
This method is described in, for example, Japanese Patent Application (OPI) Nos. 34737/78 and 163538/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") and J.C.S., Perkin I, 1862 (1980) and the like. This method, however, accompanies not a few disadvantages. In other words, an excess amount of formaldehyde over the phenol, a poisonous catalyst (e.g., SnCl.sub.2, SnCl.sub.4, Cr(acac).sub.3, etc.) or a fairly large amount of an organic amine as an additive need be used; and therefore, a complicated post-treatment and waste water treatment and the like are inevitable.
This method includes, for example, a method for using an excessive amount of potassium tertiary butoxide in dimethylformamide (as described in Angew. Chem., 86, 386 (1975)). In these methods, however, it is general that large amounts of a special solvent and base should be used, and that the yield of desired product is very low when substrates other than 2,6-disubstituted p-cresol are used.